A long-running mystery of Earth scienceconcerns the physical and dynamic connectionbetween volcanism, plutonism, magma cham-bers, layered intrusions, and the productionfrom mantle material of oceanic and continentalcrust.The basic chemical connection of frac-tionating melt from crystals and resortingcrystals again and again is clear, but volcanol-ogists and plutonists covet almost mutuallyexclusive,process-oriented sciences to explainthe compositional sequences and texturesfound in stacks of lavas and expanses of plutons.

Lavas present an extensive time series ofquenched aliquots of magma without anyclear measure of the evolutionary spatial con-text of sampling within the magmatic system.Plutons offer extensive spatial context withouta clear physical connection to the greateractive magmatic system and without a sequen-tial capturing of time; critical original texturaldetails are erased under long cooldown times.A deadly middle ground must be crossed tomeld the timescales and textures captured byvolcanism with the spatial scales and texturalprocesses of plutonism into a conceptual rep-resentation of an integrated working magmaticsystem.

A start may be a magmatic mush column(MCC),and the Ferrar Dolerites of the McMurdoDry Valleys of Antarctica may be the RosettaStone example.

A magmatic mush column (Figure 1a) is anextensive, vertically interconnected stack ofsheets and chambers extending upward throughthe lithosphere and capped by a volcaniccenter.Highly fractionated and primitive magmas,and everything in between, may coexist as,respectively, pools of nearly crystal-free mag-ma, crystal-rich magma, thick beds of cumu-lates,and open or congested (by cognate andwall debris) conduits. Dynamic solidificationfronts sheath all system boundaries, advancingin response to local geometry, thermal regime,and level of magmatic activity.

The eruptive chemical and petrographicnature hinges on the strength and duration ofthe eruption flux: the stronger and longer theflux, the more crystal-laden and primitive theeruptive.The surficial chemical impression ofthe deeper nature of the system dependsheavily on the temporal dynamics of the system.A system may appear petrologically distinctfrom one episode to the next.

The conceptual philosophy behind themeasures and processes used to gauge thesystem strongly colors how the system itself isperceived: Bowenian thinking yields Bowenianprocesses.A key to appreciating the level ofphysical and chemical integration of a MMCis to find a vertically extensive and intricatelyexposed system, revealing the spatial connec-tions between lavas, sills,feeders,and ultramafic-layered rocks.A MMC of this basic nature existsin the McMurdo Dry Valleys region of Antarctica(Figure 1b).

The Ferrar Magmatic Mush Column

The McMurdo Dry Valleys magmatic systemcontains a vertically interconnected stack offour large (~300 m) sills floored by an ultra-mafic-layered intrusion and contiguous into anextensive capping of flood basalts (KirkpatrickBasalt, Figure 1c). Unparalleled exposures ofan extensive massive tongue of large orthopy-roxene (Opx) phenocrysts in the Basement(lowermost) Sill serve as a tracer of emplace-ment and differentiation dynamics (Figures 1band 1c). A Basement Sill lobe forms the small(~700 m), beautifully layered Dais Intrusion(Figures 2 and 3).Crystal sorting has producedmassive brows of orthopyroxenite above lay-ers of anorthosite; rapid cooling has capturedhitherto unseen critical dynamic aspects ofsorting and crystallization.The production ofhighly differentiated compositions, delicatelylayered ultramafics, and all lithologies inbetween can be directly observed.

System-wide chemical fractionation is par-ticularly distinctive.The Opx tongue containsup to 20 wt % MgO with locally abundantlenses, seams, and stringers of anorthosite;

tongue thickness decreases outward from thefilling point to the distal, leading sill edges.Magma devoid of tongue sludge (chilled mar-gins and leading tips) contains ~7 wt % MgO.Similarly, the systematic loss of tongue sludgeupward through the system causes progressiveMgO depletion until it reaches ~5 wt % in theuppermost sill, forming a smooth chemicaltransition into the contemporaneous Kirkpatrickflood basalt [Fleming et al., 1995].

Virtually the same chemical transitions arethus observed vertically and laterally throughthe system and the Basement Sill.Local magmacomposition is directly related to the mechanicalgain or loss of Opx phenocrysts,which is stronglycoupled to the prevailing flow dynamics ofemplacement and eruption.A similar processhas long been noted at Kilauea, Hawaii.

The Ferrar Dolerites

The McMurdo Dry Valleys are Earth’s mostancient landscapes (Figure 1b [Denton et al.,1993]). Initially sculpted by rivers and windunder arid conditions in the early Tertiary,they have remained remarkably essentiallyunchanged, except for minor retooling byalpine glaciers. Intervalley mountains formupland plateaus supporting high buttes ofBeacon sandstone often capped and trussedby sheets of dolerite. Establishment of thenearby polar ice sheet,held at bay by massivedolerites, began about 30 million years agowhen Antarctica disconnected from SouthAmerica and the circumpolar current developed.

Early explorers seeking the South Pole hadto cross the Transantarctic Mountains to gainpolar plateau access.Geologists with R.F. Scott’sDiscovery Expedition (1901–1904) and E. H.Shackleton’s Nimrod Expedition (1907–1909)readily recognized the abundance of thickdark bands of dolerite within massive basalexpanses of pink granite and extensive uppersections of tan sandstones. Modern petrologicreports began with Warren Hamilton in 1965and Bernard Gunn in 1966 in association withthe International Geophysical Year 1957–1958.

Dolerites reflect Gondwana breakup; theDry Valleys are the almost unfaulted andundeformed west rift flank.The sills are of twomain petrologic types (Figure 1c): all doleritecontaining Opx phenocrysts shows sortingand layering of some kind.All dolerite devoidof Opx phenocrysts is uniform and featureless.This fundamental distinction cannot beemphasized too strongly,coloring all petrologicalaspects of this system.

VOLUME 85 NUMBER 47

23 NOVEMBER 2004

PAGES 497–508

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PAGES 497, 502

A Magmatic Mush ColumnRosetta Stone:The McMurdo Dry Valleys of Antarctica

BY BRUCE MARSH

Eos,Vol. 85, No. 47, 23 November 2004

Fig.1.(a) A magmatic mush column as a seriesof high aspect ratio interconnected magmachambers.Color hotness portrays magma temperature; small black squares depict largecrystals of olivine or orthopyroxene.Magmacomposition within the system is characterizedby CaO versus MgO (wt %) for lavas fromHawaii. First-order degree of primitiveness isgauged by MgO content from typical mantle tooceanic and continental material.A similarvariation is found within the Ferrar magmaticsystem.The slope at higher MgO reflects olivinecontrol at Hawaii versus orthopyroxene in theFerrar. (b) McMurdo Dry Valleys areal distribu-tion of the massive,ultramafic Basement Sill.(c) Sill emplacement sequence from ascendingmagma carrying an extensive tongue of entrainedphenocrysts. Stratigraphic relations within theMcMurdo Dry Valleys show interconnectednessof sills as a mush column and the spatial char-acter of the Opx tongue.

The Opx Tongue and Kinetic Sieving

Dense entrained phenocrysts, like Opx,migrate during magma ascent inward fromthe conduit walls, sink back from the leadingedge, and become a sorted slurry, sludge, orsuspension of solids sheathed in crystal-free,highly fractionated melt [Simkin, 1967; Marsh,1996] (Figures 1c and 2).The longer the ascent,the better sorted becomes the tongue of crystals.

As magma ascent stalls and propagates laterally into a sill, the most fractionated,crystal-free melt freezes into chilled margins,and solidification fronts propagate inwardeverywhere along the margins.The tongue ofphenocrysts is progressively captured by thesolidification fronts as it arrives and spreadsoutward.

A repose or hiatus in the injection process,as expected in any magmatic process, allowstongue crystals to sediment, further sort, andbe deposited into the lower solidificationfront. Flow resumption remobilizes throughshear the mobile tongue crystals and throughthis cycle goes on to form compositional andmodal steps in the final sill structure [Charrierand Marsh, 2004].The Opx tongue fine struc-ture records the emplacement dynamics.

By and large, the filling points of major sillsare unknown.The thickness and extent of theOpx tongue records Basement Sill filling.Every-where outward from Bull Pass in the OlympusRange, the tongue thins (Figures 1b, 1c, and2); emplacement occurred as an expansiveplume of magma.Yet, the detailed magmadynamics is distinctive in all directions. Eastover 7 km,the sill thins dramatically into a 1-cm

wispy dike; after 4 km, the Opx tongue similarlythins into a blunt tip, beyond which sill com-position is uniformly 7 (wt %) MgO (Figure 2).Northward, the tongue continues massivelywith distinct compositional reposes.Southward,the sill is a series of laterally coalescing lobes(Figure 3), similar to the leading edge of mas-sive lava.

Each lobe also pushes against the overlyingPeneplain Sill with some lobes appearing pen-etrative into a NE-SW fissure-style feeder, form-ing at Bull Pass (Figure 2),a fir-tree-like structurein the fashion of a small MMC.Westward, thelobate sequence dips into the lowest point ofWright Valley,becoming increasingly organizedinto gross layers of anorthosite and pyroxenite,culminating in the Dais Intrusion (Figure 3).

The Dais Layered Intrusion

Kinetic sieving during shear pervasively sortsplagioclase into thin seams and stringers ofanorthosite within the Opx tongue [e.g., Savageand Lun, 1988].A grain-supported matrix ofcoarse crystals sieves much smaller crystalsduring avalanching. Every variety of layeringappears, involving tiny (0.1 mm) grains of pla-gioclase and pyroxene and much larger (1–25mm) grains and cumulate clusters of pyroxene.

Compositional profiles indicate both modaland cryptic layering (Figure 3).Tiny crystalshave drained, leaving massive pyroxenite browscapping 20 m anorthositic layers.Apparentanorthosites show a 50–50 mix of tiny Opxand plagioclase, with Opx grains annealinginto large grains containing abundant plagio-clase, thus growing an internal sieve structure

within already sieved crystals, self-organizinginto layers of coarse pyroxene and cleaner(tan,buff, and white) “sandstone”anorthosites.In the purest anorthosites, further annealingforms patches of massive,optically continuousplagioclase. Rapid cooldown has quenchedand preserved these hitherto unobserved tex-tures before loss to annealing through protractedcooling, which enhances layering [Boudreau,1994].

A Magmatic Rosetta Stone:Lessons Learned

Magmatic systems are highly integratedphysically, chemically, and spatially. At everyscale, the system is characterized by physicalprocesses buttressed by chemical processes.In the conventional sense, there is no primarymagma,only a primary process.The local size,shape, and age of the system coupled withmagma crystallinity, integrated flux, flushingfrequency,and nature of wall rock involvementdetermines the local and system-wide products.

Layering starts as the unavoidable physicalprocess of sorting and ordering of crystalswarms chaotically entrained by periodicflows at geometrically convenient locations,and ends as the chemical process of texturalannealing.There are probably no magmas thatbegin life as clean, crystal-free fluids, then,ascend, grow, and fractionate crystals andbecome something markedly different. Rather,magmatic diversity is the continual attempt ofoften haphazardly juxtaposed crystals andmelt (concomitant and otherwise) to reachphysical and chemical equilibrium.The con-venient convention of magmatists of inventing“source”and “high level”convoluted processesto explain magmatic diversity is specious.

Acknowledgments

Conversations and/or field assistance fromB. Gunn, G. Denton, D. Marchant, M. Zieg,J. Phillip, D. Noe, R. Flanagan-Brown, M.Weiss,B. Phillips,A. Charrier,T. Hersum, and others,including pilots and McMurdo support staff,are greatly appreciated.

This work was supported by U.S. NationalScience Foundation grants OPP 9814332 andEAR-0207254.

References

Boudreau,A. (1994), Mineral segregation duringcrystal aging in two-crystal, two-component systems, S.Afr. J.Geol, 4, 473–485.

Charrier,A.,and B.D.Marsh (2004),Sill emplacementdynamics from regional flow sorting of OPX phe-nocrysts, Basement Sill, McMurdo Dry Valleys,Antarctica, Eos,Trans.AGU, Joint Assembly Suppl.,85(17),Abstract v42A-03.

Denton,G.H.,D.E.Sugden,D.R.Marchant,B.L.Hall.,and T. I.Wilch (1993), East Antarctic Ice Sheet sen-sitivity to Pliocene climatic change from a Dry Val-leys perspective, Geogr. Ann., 75(4), 155–204.

Fleming,T. H., K.A. Foland, and D. H. Elliott (1995),Isotopic and chemical constraints on the crustalevolution and source signature of Ferrar magmas,North Victoria Land,Antarctica, Contrib.Mineral.Petrol., 121, 217–236.

Eos,Vol. 85, No. 47, 23 November 2004

Fig.2. Looking north through Bull Pass showing the Basement Sill, its Opx tongue,and its con-nection to the overlying Peneplain Sill; inset shows the sill continuation to the right.Vertical com-positional profiles delineate the Opx tongue,which diminishes in intensity as the sill itself thinsand withers to the east. Each MgO profile has a maximum of 20 wt %; the background MgO is7 wt % in the chilled margins and leading sill tip.

Eos,Vol. 85, No. 47, 23 November 2004

Fig.3.Wright Valley south wall panoramic (upper)from Bull Pass.Mountain peaks are PeneplainSill overlying bulbous lobes of Basement Sill,outlined in color to depict temperature duringemplacement and propagation southward.Tothe right,Basement Sill plunges to the valleyfloor to form the Dais Layered Intrusion (bottomphoto).Cryptic and strong modal layering isevident in chemical profiles of CaO and MgOat 5-m intervals.Thick (white) bands of plagio-clase-rich rock are overlain by massive browsof pyroxenite.Bottom inset exemplifies small-scale layering.The variation of CaO with MgO(inset) mimics the same variation observed forthe entire Ferrar system; pyroxenites form theMgO-rich trend and plagioclase-rich rocks formthe highly fractionated CaO-rich trend.Thismagmatic sequence can be viewed as mantle-like at the bottom and continental-like at thetop; intermediate changes from mantle-like tooceanic crust-like take place over a few metersdue to pronounced modal sorting (yellow andgreen circles).

Marsh, B. D. (1996), Solidification fronts andmagmatic evolution, Mineral.Mag., 60, 5–40.

Savage, S. B., and C. K. K. Lun (1988), Particle sizesegregation in inclined chute flow of drycohesionless granular solids, J. Fluid Mech., 189,311–335.

Simkin,T. (1967), Flow differentiation in the picriicsills of North Skye, in Ultramafic and RelatedRocks, edited by P. J.Wyllie, pp. 64–69, John Wiley,Hoboken, N. J.

Author Information

Bruce Marsh, Johns Hopkins University,Baltimore, Md.; E-mail: bmarsh@jhu.edu